Implications for dark energy:
In the immediate aftermath of GW170817 the consequences of tight bounds on \(c_{\rm GW}\)  – \(|\delta c_{\rm GW}| \lesssim 10^{-15}\)  – on dark energy theories were explored in detail (see e.g. [1]}, [2]}, [3]}, [4]} and references therein), assuming these bounds can simply be ported to much lower cosmological frequencies. The resulting conclusions would be significantly more robust in the presence of an analogous precision measurement at LISA frequencies, as discussed and forecasted here. This is because we can consistently describe both cosmological and LISA scales in dark energy theories that lead to \(c_{\rm GW}\ne c\)  on cosmological scales, while this is much more challenging for LVK scales [5]}.
